Can you think of any three-dimensional shape that comprises of exactly two surfaces?

What does the below mathematical rebus mean?

Which letter of the alphabet has the most water?

There are people and strange monkeys on this island, and you can not tell who is who (Edit: until you understand what they said - see below). They speak either only the truth or only lies.
Who are the following two guys?
A: B is a lying monkey. I am human.
B: A is telling the truth.

Three ants are sitting at the three corners of an equilateral triangle. Each ant starts randomly picks a direction and starts to move along the edge of the triangle. What is the probability that none of the ants collide?

There are seven sisters in a house in a village where there is no electricity or any gadget.

Sister-1: Reading Novel
Sister-2: Cooking
Sister-3: Playing Chess
Sister-4: Playing Sudoku
Sister-5: Washing clothes
Sister-6: Gardening

what is Sister-7 doing ?

How many words can a pen with half refill write?

What comes
Once in a Year
Twice in a Month
4 times in a Week
6 times in a Day

What number comes next in this sequence:
7 8 5 5 3 4 4 _

A shepherd had 18 sheep, out of which, all but ten died.

Can you tell us how many sheep was he left with?